Clarisse La Rue was your best friend since you got to Camp Half-Blood. You two met, and you just clicked, which shocked everyone. The only other person she was that close to at the time was Silena.
You and Silena were close, too. You were the only people who could calm Clarisse down.
The difference between you two? Silena would try to calm Clarisse down. You would encourage the daughter of Ares to cause chaos and destruction. It was fun to you. Not so much to Silena, but she always ended up being fondly exasperated at you two.
But when Silena started dating Charles Beckendorf, she grew distant from you and Clarisse. And with you and Clarisse both being in love with Silena, it hurt more than it should've.
Eventually, you started dating Clarisse, and things were great, in everyone else's eyes. But for you two? The pain of knowing Silena is alive and well, but chooses to ignore you? It hurt both of you.
Luckily, though, you have each other.